The invention relates to an electrical contact having a contact head movable toward and away from a base and connected to the base by an axially collapsible flexible connector surrounded by a helical compression spring urging the contact head away from the base.
Such electrical contacts are well known. In use the contact is situated in a guide relative to which the contact head is movable. The contact head in its extended state rests against a shoulder in the guide and is movable away from the shoulder upon engagement with an opposing contact, which may itself be rigid or elastic. The movable contact head is pressed against the shoulder or the opposing contact by the action of the spring surrounding the flexible connector.
These electrical contacts represent a great improvement as compared to the prior rigid connectors, but special care should be applied to reduce the ohmic voltage drop of the current traversing them as much as possible. This involves proper selection of the materials (brass, copper, silver pellet on the contact tip)--improvement of the intermediate contacts (the connections between the flexible connector and the contact head and the base etc.). Since the apparatus of these electrical contacts, hereinafter referred to as axial thrust contacts, these various features have formed the objects of numerous improvements some of which have been patented.
It is also of importance to establish a particular pressure between the opposed contacts. In practice, this should in the first place be adequate to assure a satisfactory contact on the one hand. According to the laws of Amonton state, the area of the true contact surface is proportional to the force applying the two conductors against each other, this finding being explained on the basis of flattening of small peaks to be on all surfaces even the most highly polished. Secondly it is advantageous that the thrust should be practically constant throughout the action, to prevent excessive force upon engagement. Use should consequently be made of a prestressed spring rather than a spring near its extended state so that an appreciable residual elastic force should be present when the contacts are apart. It is in order to prevent deterioration of the flexible connector and of its fittings, that the contact head abuts the shoulder of its guiding well when the opposed contacts are apart. In this manner, the flexible connector is never in the maximum elongation position.
A tubular metal braid or hollow braid whereof the deformation which by inflation allows a substantial longitudinal collapse of the order of 30%, has hitherto been used as a flexible connector in known axial thrust contacts. A connector of this kind has a number of shortcomings.
The properties of the connector should be selected as a function of the current intensity in accordance with the standards adopted and, as the cross section is annular it is quite obvious that the diameter of the middle portion of the conductor at rest is appreciably greater than that which a non-tubular cable of identical useful cross section would have. However, any increase of diameter of the connector implies an increase of the diameter of the spring and of the surrounding guide and thus an increase in the bulk and cost price of the device. This then leads to selecting the minimum value compatible with the current intensity for the useful cross section which results in increased Joule heating and a reduction in mechanical strength.
Apart from the total mechanical strength which decreases with each action, each strand weakening by undergoing an increase (upon engagement) and then a reduction (upon separation) of its radius or curvature, account should also be taken of the wear caused by friction between strands. There are in effect two sources of wear. The outer strands rub against the turns of the spring upon each expansion, unless of course a spring of sufficiently large diameter is selected, but this leads back to the disadvantage of bulk and of an increase in cost price. Occasionally, it even happens that strands are gripped and finally severed between the consecutive turns of the spring. Secondly the interlaced strands of the braid rub on each other. Strands are thus severed after repeated operation, which obviously results in an increase in the electrical resistance of the connector and to an increase of the voltage drop across it. As a rule, it is estimated that a contact should be scrapped when the increase in resistance reaches 10%.
In the optimum conditions regarding materials, the length/diameter ratio and the deflection (difference between the length at rest and the length under compression) the tubular braidings allow approximately 5000 operations before the increase in the electrical resistance reaches 10%.
This maximum number of operations is insufficient in many cases, and it is desirable to prolong the tip of the axial thrust contact.